1. Field of the Invention
The invention relates to fiber optic sensors. More particularly, the invention relates to methods and apparatus for mechanically enhancing the sensitivity of transversely loaded fiber optic sensors and for converting pressure to transverse strain on a fiber optic sensor.
2. State of the Art
Fiber optic sensor technology has developed concurrently with fiber optic telecommunication technology. The physical aspects of optical fibers which enable them to act as wave guides for light are affected by environmental influences such as temperature, pressure, and strain. These aspects of optical fibers which may be considered a disadvantage to the telecommunications industry are an important advantage to the fiber optic sensor industry.
Optical fibers, whether used in telecommunications or as environmental sensors, generally include a cylindrical core, a concentric cylindrical cladding surrounding the core, and a concentric cylindrical protective jacket or buffer surrounding the cladding. The core is made of transparent glass or plastic having a certain index of refraction. The cladding is also made of transparent glass or plastic, but having a different, smaller, index of refraction. The ability of the optical fiber to act as a bendable waveguide is largely determined by the relative refractive indices of the core and the cladding.
The refractive index of a transparent medium is the ratio of the velocity of light in a vacuum to the velocity of light in the medium. As a beam of light enters a medium, the change in velocity causes the beam to change direction. More specifically, as a beam of light travels from one medium into another medium, the beam changes direction at the interface of the two media. In addition to changing direction at the interface of two media, a portion of the incident beam is reflected at the interface such that the energy of the beam travelling through the second medium is diminished (the sum of the energy of the refracted and reflected beams must equal the energy of the incident beam). The angles of reflection and refraction can be predicted using Snell's law if the refractive indices of both media are known.
By altering the indices of refraction of two adjacent media, the angle of refraction and the angle of reflection of a beam travelling toward the interface of the two media can be altered such that the intensity of the light entering the second medium approaches zero and substantially all of the light is reflected at the interface. Conversely, for any two transparent media, there is a critical angle of incidence at their interface at or below which substantially all of the incident light will be reflected. This phenomenon, known as total internal reflection, is applied in choosing the refractive indices of the core and the cladding in optical fibers so that light may propagate through the core of the fiber with minimal power loss.
Many other factors affect the propagation of light through the fiber optic core, including the dimensions of the core and the cladding, the wavelength of the light, the magnetic field vectors of the light and electrical field vectors of the light. In addition, many of the physical laws used to determine the ideal propagation of light through a wave guide (optical fiber) assume an "ideal" wave guide, i.e. a straight wave guide with perfect symmetry and no imperfections. For example, the diameter of the core will determine whether the fiber optic is "single mode" or "multimode". The terms single mode and multimode refer to the dimensional orientation of rays propagating through the fiber. Single mode fibers have a core with a relatively small diameter (2-12 microns) and support only one spatial mode of propagation. Multimode fibers have a core with a relatively large diameter (25-75 microns) and permit non-axial rays or modes to propagate through the core. The so-called single mode fibers are actually two mode fibers in the sense that there are two different states of optical polarization that can be propagated through the core. In an ideal, straight, imperfection-free fiber with perfect circular symmetry, the propagation velocity of light is independent of the direction of polarization.
A fiber with an elliptical core will have two preferred directions of polarization (along the major axis and along the minor axis). Linearly polarized light injected into the fiber at any other direction of polarization will propagate in two separate modes that travel at slightly different velocities. This type of fiber is said to have a "modal birefringence". In a real fiber of this type, even ideally polarized light will couple into the other mode due to imperfections in the core-cladding interface, index of refraction fluctuations, and other mechanisms. Static and dynamic changes in polarization may occur along the entire length of the fiber. Over a given distance, the phases of the two modes will pass through an entire cycle of being in phase and out of phase. This distance is known as the "beat length". A long beat length is associated with a small birefringence and a short beat length is associated with a large birefringence. Birefringent optical fibers are also known as "polarization preserving fibers" or "polarization maintaining (PM) fibers". Birefringence is achieved by providing a core with an elliptical cross section or by providing circular core with a cladding which induces stress on the core. For example, the cladding may be provided with two parallel stress members having longitudinal axes which lie in the same plane as the axis of the core.
As mentioned above, fiber optic sensors employ the fact that environmental effects can alter the amplitude, phase, frequency, spectral content, or polarization of light propagated through an optical fiber. The primary advantages of fiber optic sensors include their ability to be light weight, very small, passive, energy efficient, rugged, and immune to electromagnetic interference. In addition, fiber optic sensors have the potential for very high sensitivity, large dynamic range, and wide bandwidth. Further, a certain class of fiber sensors may be distributed or multiplexed along a length of fiber. They may also be embedded into materials.
State of the art fiber optic sensors can be classified as either "extrinsic" or "intrinsic". Extrinsic sensors rely on some other device being coupled to the fiber optic in order to translate environmental effects into changes in the properties of the light in the fiber optic. Intrinsic sensors rely only on the properties of the optical fiber in order to measure ambient environmental effects. Known fiber optic sensors include linear position sensors, rotational position sensors, fluid level sensors, temperature sensors, strain gauges, fiber optic gyroscopes, and pressure sensors.
One type of fiber optic pressure sensor takes advantage of the fact that ambient pressure places a strain on the jacket of an optical fiber which strains the cladding, thereby straining the core and changing the birefringence of the fiber. U.S. Pat. No. 4,659,923 to Hicks, Jr. discloses a fiber optics interferometer transducer which is capable of measuring pressure. The device generally includes a single mode fiber optic, a light source, a first polarizer, a second polarizer, and a light intensity detector. The first polarizer is used to polarize the light source to an angle which is 45.degree. to the two modes of the fiber optic, whereafter the light is injected into one end of the fiber. The second polarizer is arranged at the other end of the fiber and polarizes light exiting the fiber to the same angle as the first polarizer before it is detected by the intensity detector. When a force is applied transversely to the fiber, the birefringence of the fiber changes, which changes the beat length and thus the intensity of the polarized light viewed by the intensity detector. In an alternative embodiment, a beam splitter is placed between the light source and the first polarizer, and the second polarizer is replaced by a mirror coupled to the second end of the fiber optic. According to the alternative embodiment, the detector and the source may be located at the same end of the fiber optic. No data is provided regarding the sensitivity (resolution) or dynamic range of the proposed sensors. However, it is suggested that the effects of pressure on birefringence may be too small to measure at relatively low pressures. Various structures are disclosed for mounting the fiber such that isotropic forces are converted to anisotropic forces to produce birefringence and to magnify the effect. One of the structures used for this purpose is suggested by the '923 patent and disclosed in greater detail by Jansen and Dabkiewicz in an article entitled "High Pressure Fiber Optic Sensor with Side Hole Fiber", published in SPIE Proceedings, Fiber Optic Sensors II, Vol. 798, pp. 56-60, 1987. Side hole fiber is a fiber optic having a cladding which contains two parallel holes which run the length of the fiber and are parallel to the core. The axes of the holes and the core lie in a common plane. This geometry results in converting external hydrostatic pressure into anisotropic stress at the core thereby inducing birefringence. Jansen and Dabkiewicz demonstrate a sensor having an accuracy of .+-.0.5% in the pressure range of 100-1,000 bars (10-100 MPa, 1,450-14,500 psi) and an upper limit of 2,000 bars without fiber failure. Below 100 bars, however, birefringence tends to become undetectable or nonexistent.
Another type of fiber optic sensor utilizes intra-core fiber gratings as disclosed in U.S. Pat. No. 5,380,995 to Udd et al., the complete disclosure of which is incorporated by reference herein. Intra-core Bragg gratings are formed in a fiber optic by doping an optical fiber with material such as germania and then exposing the side of the fiber to an interference pattern to produce sinusoidal variations in the refractive index of the core. Two presently known methods of providing the interference pattern are by holographic imaging and by phase mask grating. Holographic imaging utilizes two short wavelength (usually 240 nm) laser beams which are imaged through the side of a fiber core to form the interference pattern. The bright fringes of the interference pattern cause the index of refraction of the core to be "modulated" resulting in the formation of a fiber grating. Similar results are obtained using short pulses of laser light, writing fiber gratings line by line through the use of phase masks. By adjusting the fringe spacing of the interference pattern, the periodic index of refraction can be varied as desired.
U.S. Pat. No. 5,380,995 to Udd et al. teaches the use of a remote grating which is located to sense an environmental effect such as strain or temperature and a local grating which is located to be unaffected by the environmental effect. The spectral envelopes of both gratings are compared and the effects of strain and temperature on the remote grating can thereby be separated. The '995 patent also teaches the use of two overlaid fiber gratings of different wavelengths such as 1.3 and 1.5 microns to measure two environmental effects such as strain and temperature at a single point.
Still another way to separate the effects of strain and temperature is disclosed in U.S. Pat. No. 5,591,965 to Udd, the complete disclosure of which is fully incorporated by reference herein. The '965 patent teaches the use of a pair of gratings written in substantially the same location in a birefringent fiber. When a birefringent fiber is provided with a grating, two spectral peaks are produced (one for each polarization axis) and temperature and longitudinal strain changes affect the peak to peak separation as well as the wavelength shift of the peaks. As taught in the '965 patent, a birefringent fiber provided with two spectrally separated gratings produces four spectral outputs (peaks). Spectral detectors such as Fabry-Perot etalons coupled to the fiber detect the four spectral outputs. The spectral outputs are analyzed and four equations are solved to determine both the temperature and the strain effects on the fiber.
While neither '995 nor the '965 patent specifically refers to pressure sensors, it has been demonstrated that an ultrahigh hydrostatic pressure induces fractional changes in the physical length of a fiber optic and thus induces a fractional change in the Bragg wavelength of a grating incorporated in the fiber core. For example, M. G. Xu et al., Optical In-Fibre Grating High Pressure Sensor, Electron. Lett., Vol. 29, No. 4, pp. 398-399 (1993), demonstrates how a fiber optic Bragg grating sensor can be used to measure very high pressure. In particular, the Xu et al. paper demonstrates a simple in-fiber grating sensor which exhibits a linear Bragg wavelength shift of 3.04.times.10.sup.-3 mm/MPa. The authors specifically state that far more compensation for the effects of temperature is necessary for their sensor to be valuable and that the real advantage of their sensor is only evident at ultrahigh pressure.
3. Related Inventions
Parent application Ser. No. 08/707,861, entitled "Transverse Strain Measurements Using Fiber Optic Gratings Based Sensors", filed Sep. 9, 1996, now U.S. Pat. No. 5,828,059 discloses a system and method for sensing the application of transverse stress to an optical fiber having an optical grating in its core. When the fiber is coupled to a light source and the optical grating is transversely stressed, the grating produces a reflection or transmission that has two peaks (or two minimums) in its frequency spectrum and the spectral spacing between the peaks (or minimums) is indicative of the transverse force applied to the fiber. The spectral spacing between the peaks (or minimums) is substantially unaffected by temperature. In addition, the spectral location of the two peaks (or minimums) can be used to calculate the temperature of the grating. Optical fibers with multiple gratings are also disclosed wherein simultaneous measurements of temperature and pressure may be made at different locations along the length of a fiber. According to one embodiment, fiber gratings are written into circularly symmetric single mode optical fiber. The advantages of this embodiment is that low cost telecommunications grade optical fiber can be used and the symmetry of the fiber results in transverse sensitivity which is independent of the loading direction. A disadvantage of this embodiment is that for small transverse loads, the peak to peak separation may be extremely difficult to measure as the peaks may be buried in noise.
Parent application Ser. No. 08/888,566, entitled "Fiber Optic Pressure Transducer and Pressure Sensing Systems filed Jul. 7, 1997, now U.S. Pat. No. 5,841,131 discloses a fiber optic pressure transducer having enhanced resolution and dynamic range which includes a fiber optic core having one or more gratings written onto it, a birefringence structure for enhancing the birefringence of the core, and a structure for converting isotropic pressure forces to anisotropic forces on the fiber core. Several different embodiments of prestressing structure are disclosed (both extrinsic and intrinsic). Several different embodiments of structure (both extrinsic and intrinsic) for converting isotropic pressure to anisotropic pressure are also disclosed.
While many advances have been made in improving fiber optic sensors, the sensitivity and dynamic range of strain and particularly pressure sensors is still limited to resolving relatively large changes in relatively large forces.